1. Field of the Invention
The present invention relates to high-frequency semiconductor devices and radio transmitter/receiver (transceiver) devices.
2. Description of the Related Art
Conventional high-frequency amplifiers for amplification of high-frequency signals at frequencies higher than or equal to 1 gigahertz (GHz) may include a transmitter amplifier adaptable for use in portable and mobile digital cellular radiotelephone terminals based on a currently available personal handy phone (PHP) systemxe2x80x94in Japan the acronym xe2x80x9cPHSxe2x80x9d is more popular, so this acronym will be used hereinafter. An exemplary configuration of such a PHS terminal will be explained as follows.
In such high-frequency amplifiers, a source-grounded or xe2x80x9ccommon-sourcexe2x80x9d amplifier using GaAs metal semiconductor field effect transistors (MESFETs) has been typically employed today. The power gain per stage is approximately 10 decibels (dB), and the use of a serial combination of multiplexe2x80x94two to fourxe2x80x94stages of such amplifiers permits the resultant circuitry to have an increased power gain ranging from 20 dB to 40 dB, more or less. This high-frequency amplifier is commercially available for use as a microwave monolithic integrated circuit (MMIC) in a growing electronics market. For reduction of cost penalties, MMICs are typically mounted or xe2x80x9cembeddedxe2x80x9d in plastic packages, which are low-cost housings.
One prior art high-frequency semiconductor device designed for use as a MMIC is shown in FIG. 2. FIG. 2 is a diagram showing a plan view of a MMIC 10 having a high-frequency amplifier circuitry architecture in a four-stage configuration using four MESFETs with sources grounded, where the MMIC 10 is mounted to the frame of a plastic package. The MMIC 10 is put on a metallic plate 2, called a xe2x80x9cbedxe2x80x9d among those skilled in the art to which the invention pertains.
The MMIC 10 is configured including FETs 121, . . . , 124, matching circuits MC1 to MC4, each of which consists of a capacitor and an inductor, and internal connection pads 14a to 141, 14n. 
The semiconductor device also includes xe2x80x9cexternalxe2x80x9d pins 4a-4n along with bonding wires 20a-20n. Pins 4g, 41, 4m are connected to the bed 2.
One matching circuit MC1 has a capacitor MC1a and an inductor MC1b and is connected to pads 14a, 14h. Another matching circuit MC2 having a capacitor MC2a and an inductor MC2b is connected to a pad 14i. Another matching circuit MC3 is formed of a capacitor MC3a and inductor MC3b and is coupled to a pad 14j. The remaining matching circuit MC4 with a capacitor MC4a and an inductor MC4b is coupled to a pad 14k. 
On the other hand, an FET 121 at the initial stage (first-stage FET) has its gate connected to the matching circuit MC1 and its drain connected to the matching circuit MC2 with a source connected to the pad 14b. The second-stage FET 122 has a gate connected to the matching circuit MC2, a drain connected to the matching circuit MC3, and a source coupled to the pad 14c. The third-stage FET 123 has a gate connected to the matching circuit MC3 with a drain coupled to the matching circuit MC4 and with a source coupled to the pad 14e. The fourth-stage FET 124 has its gate connected to the matching circuit MC4 and also coupled via a high resistance element to the pad 14f, a drain coupled to the pad 14n, and a source coupled to the pads 14g, 141. An output of the high-frequency semiconductor device is derived from the drain node of FET 124, i.e. the pad 14n. 
The pads 14a-14f are connected by bonding wires 20a-20f to pins 4a-4f, respectively; pads 14h-14k are connected via bonding wires 20h-20k to pins 4h-4k, respectively. The pad 14g is connected to the bed 2 via three bonding wires 20g, pad 141 is coupled to bed 2 by four bonding wires 201. As the bed is coupled to the grounded power supply in most cases, the pins 4g, 41, 4m are provided as GND-pins. The pad 14n is tied via the bonding wire 20n to the output pin 4n. 
Accordingly, pads 14g, 141, which are connected to the source of the final-stage FET 124 that is in closest proximity with the source side and thus suffers most significantly from a parasitic inductance problem, are directly connected by bonding wires to the bed 2.
While the bed 2 is typically connected to more than one GND-pin in the way stated above, an inductance does exist at the GND-pins 4g, 41, 4m shown in FIG. 2, which results in the bed 2 not being set at the xe2x80x9cidealxe2x80x9d GND in terms of high-frequency activities. Hereinafter, this GND which is potentially xe2x80x9cfloatingxe2x80x9d from the true GND at a certain impedance determinable in terms of high-frequencies will be referred to as a xe2x80x9cvirtualxe2x80x9d GND.
An explanation will next be given of what kinds of problems can occur due to the presence of the virtual GND, rather than the ideal GND. While spiral inductors MC1b, MC2b, MC3b, MC4b are mounted on the MMIC 10 together with metal-insulator-metal (MIM) capacitors MC1a-MC4a, their layout areas are significant to the extent that they occupy a major part of a limited chip area. Hence, a coupling capacitance is present between these elements and the bed with a semiconductor substrate laid or xe2x80x9csandwichedxe2x80x9d therebetween.
FIG. 3 is a pictorial representation of the semiconductor device structure to demonstrate how such extra capacitive components reside therein. As shown, a coupling capacitance Cp1 exists between a circuit element 31 in MMIC 10 and its bed 2 whereas another coupling capacitance Cp2 is between an element 32 and bed 2. Because the bed 2 is potentially xe2x80x9cfloatingxe2x80x9d in terms of high frequencies as stated previously, the element 31 and element 32 are electrically coupled together via the capacitances Cp1, Cp2. Now, if it is assumed that the bed 2 is completely floating in potential from the true GND in the worst case, the element 31 and element 32 could be coupled together via a series-connected capacitor that is formed by the capacitance components Cp1 and Cp2.
Such an undesirable capacitive coupling between or among certain elements on the MMIC 10 can result in a variety of kinds of malfunctions and operational failures. Especially, in multi-stage amplifier circuitry with a series combination or xe2x80x9ccascadexe2x80x9d connection of multiple FETs, the element-to-element capacitive coupling can result in serious problems, such as oscillation. An evaluation was done by the present inventors, described herein, which revealed that the most problematic issue lies in capacitive coupling between an input-side matching circuit of the N-th stage FET and an output-side matching circuit of its neighboring, (N+1) th stage FET (where, N is a natural number).
Suppose that an amplifier includes a serial combination of four stages of FETs as shown in FIG. 4. As shown in FIG. 4, four matching circuits MC1, MC2, MC3, MC4 are present between an input stage and output stage of such amplifier. Respective of the matching circuits is configured from a spiral inductor and an MIM capacitor, which can be capacitively coupled together through the bed 2 laid therebetween. For clarity purposes, consider the coupling between the matching circuits MC1, MC3 only, which will be represented as a coupling capacitance Cf.
FIG. 5 shows the stability of the circuitry in FIG. 4, as obtained by simulation. An amplifier tested is of the class with a gain of 40 dB in the 1.9 GHz band. Supposing that the frequency in question as to the stability falls within a range of from 0.1 GHz to 10 GHz; then, it has been investigated how a minimal value (Kmin) of a stability factor K varies depending upon the feedback capacitance Cf in this frequency range. The result is shown in FIG. 5. As is apparent from FIG. 5, when Cf goes beyond 13 femto-farads (fF), the value Kmin becomes less than 1 (i.e., Kmin less than 1) resulting in dissatisfaction of the absolute stability criteria involved. When Cf further increases to exceed 17 fF, the value Kmin is less than zero (Kmin less than 0), which results in oscillation in a system of 50-Ohm (xcexa9) input/output impedance. Incidentally, those inductors and capacitors for use in the MMIC 10 inherently have a capacitance relative to the bed, which capacitance is at least in the order of magnitude of several tens of fF. Thus, there must apparently exist the possibility that the Cf value increases up to about 18 fF or greater, which in turn leads to the creation of a problem that the circuitry is hardly stable in operation. Prior art approaches to achievement of the intended stability are merely to reduce the gain of amplifier circuitry, per se.
Another problem faced with the prior art is that in the case where the MMIC is an oscillator, any desired oscillation frequency is by no means obtainable even in view of the element-to-element or xe2x80x9cinterelementxe2x80x9d capacitance via the bed.
It has been described that prior art high-frequency semiconductor devices are encountered with difficulties in achieving a desired circuit operation unless the inter-element capacitance via the bed is taken into careful consideration, and sometimes faced with a more serious problem as to the inability to attain any desired operation even after consideration of such inter-element capacitance.
The present invention has been made in view of the foregoing technical background, and its primary objective is to provide a new and improved high-frequency semiconductor device with an MMIC capable of achieving increased performance.
An object of the present invention is to address the above described and other shortcomings of conventional devices. As there are various facets of the present invention, only selected features of the present invention are discussed below in this section of the document. However, a more complete discussion of the present invention is provided in subsequent sections.
In order to achieve the above-described and other objects, a high-frequency semiconductor device is provided with the following features. A microwave monolithic integrated circuit is included and has a circuit element having first and second passive element sections each having at least one passive element, and a transistor with a gate connected to the first passive element section and a drain connected to the second passive element section. A bed is mounted on the microwave monolithic integrated circuit, and is made of a conductive plate having at least one opening. The opening is provided at a position of the bed underlying at least one of the first passive element section and the second passive element section.
It is desirable that the first passive element section be connected to an input side of the transistor whereas the second passive element section be connected to an output side of the transistor.
It is also desirable that the opening be formed at a position of the bed underlying one of the first and second passive element sections which is placed on an input side of the transistor.
It is desirable that the opening be provided at a position of the bed selectively underlying at least one of the first passive element section and the second passive element section.
In addition, a high-frequency semiconductor device in accordance with this invention includes the following features. A microwave monolithic integrated circuit is included that has an amplifier circuit with a cascade connection of a plurality of amplifying elements, each having a matching circuit and a transistor with a gate connected to the matching circuit. A bed is mounted to the microwave monolithic integrated circuit and is made of a conductive plate having at least one opening. It is also desirable that the gate of the transistor be connected to the matching circuit on an input side with a drain of the transistor coupled to the matching circuit on an output side. It is desirable that the opening be formed at a position of the bed underlying at least one of the matching circuit on the input side of one of the amplifying elements with a maximal gain and the matching circuit on the output side thereof. It is desirable that the opening be formed at a position of the bed underlying the matching circuit on the input side of an initial stage one of the amplifying elements. It is desirable that each matching circuit have at least one passive element. Also desirably, the bed selectively has at least one opening under at least one of the matching circuits.
In the respective embodiments stated above, it is desirable that the passive element be at least one of a capacitor and an inductor. It is also desirable that the bed with the microwave monolithic integrated circuit mounted thereon be sealed by a resin material. Further, it is desirable that the microwave monolithic integrated circuit be securely fixed to the bed by use of adhesive made of nonconductive materials.
Furthermore, a radio transmitter/receiver device in accordance with the present invention includes an antenna unit, a high-frequency switch unit for switching between transmission and reception of the antenna unit, a first amplifier unit for amplifying a reception signal sent from the high-frequency switch unit, a receiver circuit unit for processing the amplified reception signal passed from the first amplifier unit, a transmitter circuit unit for sending forth a transmission signal, and a second amplifier unit for amplifying the transmission signal transferred from the transmitter circuit unit and for sending forth the amplified transmission signal toward the high-frequency switch unit.
The above-noted high-frequency semiconductor device is used in at least one of the first amplifier unit and second amplifier unit.